A. Background of Related Art
In the prior art, it was known that cushioning material could be formed from elastomeric gels (see, e.g., U.S. Pat. No. 5,334,646, issued on Aug. 2, 1994 in the name of John Y. Chen), foam rubber, lubricated microspheres (see, U.S. Pat. Nos. 5,421,874 and 5,549,743, issued on Jun. 6, 1995 and Aug. 27, 1996, respectively, both in the name of Tony M. Pearce) and other substances.
The addition of adhesives to elastomeric materials is also well known. For example, REGALREZ.RTM. resins have been suggested for use in combination with KRATON.RTM. thermoplastic rubbers (see U.S. Pat. No. 4,833,193, col. 2, lines 22-27, issued on May 23, 1989 in the name of David L. Sieverding). Many of those combinations include a substantial amount of KRATON.RTM.. Some REGALREZ.RTM./KRATON.RTM. combinations of the prior art even include greater amounts of KRATON.RTM. than REGALREZ.RTM.. Many prior art adhesive elastomeric materials employ small amounts of resins.
U.S. Pat. No. 4,833,193, issued on May 23, 1989 in the name of Sieverding, which is hereby incorporated by reference, discusses pressure sensitive adhesives. Applicant believes that the formulations disclosed in the '193 patent is the closest prior art to the formulations of his invention. It is important to note, however, that Sieverding's work was primarily directed to adhesives, not cushioning. The adhesive materials of that patent contain from about 2 to about 40 weight percent triblock copolymer of the general configuration A-B-A, alone or in combination with a diblock copolymer (col. 12, lines 8-14), at least 20 weight percent of a low molecular weight resin (col. 12, lines 15-18) and up to about 80 weight percent mineral oil having a viscosity of about 200 to about 1,200 (col. 12, lines 19-21).
Sieverding's triblock copolymers are SEBS copolymers, such as those sold by Shell Chemical Company of Houston, Tex. as KRATON.RTM. G1651 (col. 12, lines 38-40 and Table, col. 16, line 1 to col. 20, line 42). Sieverding's most preferred high molecular weight SEBS has a styrene to rubber ratio (S:EB or A:B) of about 0.48 to about 0.52 (i.e., about 48:52 to about 52:48)(col. 12, lines 40-45).
Sieverding's preferred tackifying resins are low molecular weight resins commercially available under the trade names REGALREZ.RTM. 1018 and 1033, both of which are manufactured by Hercules Incorporated of Wilmington, Del. (col. 13, lines 23, 31-33). The '193 patent states that the REGALREZ.RTM. resins desired for use in that material are liquid at room temperature (col. 3, lines 50-52). Sieverding's most preferred resin, REGALREZ.RTM. 1018, has an average molecular weight in the range of about 375 to about 430 (col. 13, lines 34-37). The Sieverding '193 patent also teaches the use of a single type of tackifying resin in high concentration in his materials (col. 13, lines 40-55).
The '193 patent does not disclose the tensile strength of the materials described therein. Because Sieverding's preferred triblock copolymers are SEBS copolymers, Applicant believes that the tensile strength of the materials of the '193 patent are low. This is because Applicant has found SEBS to be significantly inferior to his preferred polystyrene-hydrogenated poly(isoprene+butadiene)-polystyrene (S-(I+B)-S or S-(B+I)-S) copolymers. In some uses of his material, Sieverding prefers the use of mineral oil in addition to REGALREZ.RTM. 1018, which significantly decreases the visco-elastic properties of the materials. Further, Applicant also believes that Sieverding's material is inadequate for many cushioning and other applications since he prefers to use only one type of tackifying resin (Table, col. 15, line 49 to col. 20, line 43) and because of the narrow softening point ranges of tackifying resins useful in his material (col. 13, lines 40-55). In other words, various characteristics of Sieverding's material, including but not limited to softness and rebound rate, cannot be tailored for any given elastomer to plasticizer ratio. This may not be important in the field of adhesives, but it makes Sieverding's material impractical for wide application in the cushioning art.
B. Chemistry of Plasticizer-Extended Elastomers.
A basic discussion of the chemical principles underlying the characteristics and performance of plasticizer-extended elastomers is provided below to orient the reader for the later discussion of the particular chemical aspects of the invention.
The materials of the present invention are a composition primarily of triblock copolymers and plasticizers, both of which are commonly referred to as hydrocarbons. Hydrocarbons are elements which are made up mainly of Carbon (C) and Hydrogen (H) atoms. Examples of hydrocarbons include gasoline, oil, plastic and other petroleum derivatives.
Referring to FIG. 1a, it can be seen that a carbon atom 110 typically has four covalent bonding sites ".cndot.". FIG. 1b shows a hydrogen atom 112, which has only one covalent bonding site .cndot.. With reference to FIG. 1c, which represents a four-carbon molecule called butane, a "covalent" bond, represented at 116 as "-", is basically a very strong attraction between adjacent atoms. More specifically, a covalent bond is the linkage of two atoms by the sharing of two electrons, one contributed by each of the atoms. For example, the first carbon atom 118 of a butane molecule 114 shares an electron with each of three hydrogen atoms 120, 122 and 124, represented as covalent bonds 121, 123 and 125, respectively, accounting for three of carbon atom 118's available electrons. The final electron is shared with the second carbon atom 126, forming covalent bond 127. When atoms are covalently bound to one another, the atom-to-atom covalent bond combination makes up a molecule such as butane 114. An understanding of hydrocarbons, the atoms that make hydrocarbons and the bonds that connect those atoms is important because it provides a basis for understanding the structure and interaction of each of the components of the present invention.
As mentioned above, the present invention utilizes triblock copolymers. With reference to FIGS. 2a and 2b, a triblock copolymer is shown. Triblock copolymers 210 are so named because they each have three blocks--two endblocks 212 and 214 and a midblock 216. If it were possible to grasp the ends of a triblock copolymer molecule and stretch them apart, each triblock copolymer would have a string-like appearance (as in FIG. 2a), with an endblock being located at each end and the midblock between the two endblocks.
FIG. 3a depicts the preferred endblocks of the copolymer used in the present invention, which are known as monoalkenylarene polymers 310. Breaking the term "monoalkenylarene" into its component parts is helpful in understanding the structure and function of the endblocks. "Aryl" refers to what is known as an aromatic ring bonded to another hydrocarbon group. Referring now to FIG. 3b, benzene 312, one type of aromatic ring, is made up of six carbon molecules 314, 316, 318, 320, 322 and 324 bound together in a ring-like formation. Due to the ring structure, each of the carbon atoms is bound to two adjacent carbon atoms. This is possible because each carbon atom has four bonding sites. In addition, each carbon atom C of a benzene molecule is bound to only one hydrogen atom H. The remaining bonding site on each carbon atom C is used up in a double covalent bond 326, 327, which is referred to as a double bond. Because each carbon atom has only four bonding sites, double bonding in an aromatic ring occurs between a first carbon and only one of the two adjacent carbons. Thus, single bonds 116 and double bonds 326 alternate around the benzene molecule 312. With reference to FIG. 3c, in an aryl group 328, one of the carbons 330 is not bound to a hydrogen atom, which frees up a bonding site R for the aryl group to bond to an atom or group other than a hydrogen atom.
Turning now to FIG. 3d, "alkenyl" 332 refers to a hydrocarbon group made up of only carbon and hydrogen atoms, wherein at least one of the carbon-to-carbon bonds is a double bond 334 and the hydrocarbon group is connected to another group of atoms R', where R' represents the remainder of the hydrocarbon molecule and can include a single hydrogen atom. Specifically, the "en" signifies that a double bond is present between at least one pair of carbons. The "yl" means that the hydrocarbon is attached to another group of atoms. For example, FIG. 3e shows a two carbon group having a double bond between the carbons, which is called ethenyl 336. Similarly, FIG. 3f illustrates a three carbon group having a double bond between two of the carbons, which is called propenyl 338. Referring again to FIG. 3a, in a monoalkenylarene such as styrene, a carbon 340 of an alkenyl group 332 is bonded to the aryl group 328 at carbon atom 330, which has a free bonding site. In reference to FIG. 3c, aryl group 328 is part of a monoalkenylarene molecule when R is an alkenyl group. The "mono" of monoalkenylarene explains that only one alkenyl group is bonded to the aryl group.
The monoalkenylarene end blocks of a triblock copolymer are polymerized. Polymerization is the process whereby monomers are connected in a chain-like fashion to form a polymer. FIG. 4a depicts a polymer 410, which is basically a large chain-like molecule formed from many repeating smaller molecules, called monomers, M1, M2, M3, etc., that are bonded together. P and P' represent the ends of the polymer, which are also made up of monomers. In the present invention, illustrated by FIG. 4b, a monoalkenylarene end block polymer 414 is a chain of monoalkenylarene molecules 416a, 416b, 416c, etc. The chain of FIG. 4b is spiral, or helical, in shape due to the bonding angles between styrene molecules. P represents an extension of the endblock polymer helix in one direction, while P' represents an extension of the endblock polymer helix in the opposite direction.
As FIG. 4c shows, monoalkenylarene molecules are attracted to one another by a force that is weaker than covalent bonding. The primary weak attraction between monoalkenylarene molecules is known as hydrophobic attraction. An example of hydrophobic attraction is the attraction of oil droplets to each other when dispersed in water. Therefore, in its natural, relaxed state at room temperature, a monoalkenylarene polymer resembles a mass of entangled string 414, as depicted in FIG. 4d. The attraction of monoalkenylarene molecules to one another creates a tendency for the endblocks to remain in an entangled state. Similarly, different monoalkenylarene polymers are attracted to each other. The importance of this phenomenon will become apparent later in this discussion.
Like the end blocks of a triblock copolymer, the midblock is also a polymer. In the invention, the preferred triblock copolymer midblock is an aliphatic hydrocarbon. Traditionally, "aliphatic" meant that a hydrocarbon was "fat like" in its chemical behavior. Referring to FIGS. 5a through 5c, which do not show the hydrogen atoms for simplicity, an "aliphatic compound" is now defined as a hydrocarbon compound which reacts like an alkane 510 (a hydrocarbon molecule having only single bonds between the carbon atoms), an alkene 512 (a hydrocarbon molecule wherein at least one of the carbon-to-carbon bonds is a double bond), 514 an alkyne (a hydrocarbon molecule having a triple covalent bond 515 between at least one pair of carbon atoms), or a derivative of one of the above.
Referring now to FIG. 5d, which omits the bound hydrogen atoms for simplicity, aliphatic hydrocarbons known as conjugated dienes 516 are depicted. These are the preferred midblock monomers used in the triblock copolymers of the present invention. A "diene" is a hydrocarbon molecule having two ("di") double bonds ("ene"). "Conjugated" means that the double bonds 518 and 520 are separated by only one single carbon-to-carbon bond 522. In comparison, FIG. 5e shows a hydrocarbon molecule having two double carbon-to-carbon bonds that are separated by two or more single bonds, 530, 532, etc., which is referred to as an "isolated diene" 524. When double bonds are conjugated, they interact with each other, providing greater stability to a hydrocarbon molecule than would the two double bonds of an isolated diene.
FIGS. 6a through 6d illustrate examples of various monomers useful in the midblock of the present invention, including molecules (monomers) such as ethylene-butylene (EB) 612, ethylene-propylene (EP) 614, butadiene (B) 616 and isoprene (I) 618. Midblocks containing isoprene and/or butadiene monomers are useful in the material of the invention in either the hydrogenated or the non-hydrogenated form. The different structures of these molecules provide them with different physical characteristics, such as differing strengths of covalent bonds between adjacent monomers. The various structures of monomer molecules also provides for different types of interaction between distant monomers on the same chain (e.g., when the midblock chain folds back on itself, distant monomers may be attracted to one another by a force weaker than covalent bonding, such as hydrophobic interaction, hydrophilic interaction, polar forces or Vander Waals forces).
Referring to FIGS. 6a and 6b, x, y and n each represent an integral number of each bracketed unit: "x" is the number of repeating ethylene (--CH2--CH2--) units, "y" is the number of repeating butylene (in FIG. 6a) or propylene (in FIG. 6b) units, and "n" is the number of repeating poly(ethylene/butylene) units. Numerous configurations are possible.
As shown in FIGS. 7a through 7d, the midblock may contain (i) only one type of monomer, EB, EP, B or I or, as FIGS. 7e and 7f illustrate, (ii) a combination of monomer types EB and EP or B and I, providing for wide variability in the physical characteristics of different midblocks made from different types or combination of types of monomers. The interaction of physical characteristics of each molecule (monomer and block) determines the physical characteristics of the tangible, visible material. In other words, the type or types of monomer molecules which make up the midblock polymer play a role in determining various characteristics of the material of which the midblock is a part.
Attributes such as strength, elongation, elasticity or visco-elasticity, softness, tackiness and plasticizer retention are, in part, determined by the type or types of midblock monomers. For example, referring again to FIG. 7a, the midblock polymer 216 of a triblock copolymer-containing material may be made up primarily or solely of ethylene-butylene monomers EB, which contribute to that material's physical character. With reference to FIG. 7e, in comparison to the material having a midblock made up solely of EB, a similar triblock-containing material, wherein the midblock polymer 216 (either hydrogenated or non-hydrogenated) of the triblocks are made up of a combination of butadiene B and isoprene I monomers, may have greatly increased strength and elongation, similar elasticity or visco-elasticity and softness, and reduced tackiness and reduced plasticizer bleed.
The monomer units of the midblock have an affinity for each other. However, the hydrophobic attraction of the midblock monomers for each other is much weaker than the non-covalent attraction of the end block monomers for one another.
Referring now to FIG. 8a, which shows a polystyrene-poly(isoprene+butadiene)-polystyrene triblock copolymer, in a complete triblock copolymer 810, each end 812 and 814 of midblock chain 216 is covalently bound to an end block 212 and 214. P and P" represent the remainder of the endblock polymers 212 and 214 respectively. P' represents the central portion of midblock polymer 216. Many billions of triblock copolymers combine to form a tangible material. The triblock copolymers are held together by the high affinity (i.e., hydrophobic attraction) that monoalkenylarene molecules have for one another. In other words, as FIG. 8b illustrates, the endblocks of each triblock copolymer molecule, each of which resemble an entangled mass of string 414, are attracted to the endblocks of another triblock copolymer. When several endblocks are attracted to each other, they form an accretion of endblocks, called a domain or a glassy center 816. Agglomeration of the endblocks occurs in a random fashion, which results in a three-dimensional network 818 of triblocks, the midblock 216 of each connecting endblocks 212 and 214 located at two different domains 816a and 816b. In addition to holding the material together, the domains of triblock copolymers also provide it with strength and rigidity.
Plasticizers are generally incorporated into a material to increase the workability, pliability and flexibility of that material. Incorporation of plasticizers into a material is known as plasticization. Chemically, plasticizers are hydrocarbon molecules which associate with the material into which they are incorporated, as represented in FIG. 9a. In the present invention, plasticizer molecules 910 associate with the triblock copolymer 210, and increase its workability, softness, elongation and elasticity or visco-elasticity. Depending upon the type of plasticizer used, the plasticizer molecules associate with either the endblocks, the midblock, or both. For reasons that will soon become apparent, Applicant prefers plasticizers 910 which associate primarily with midblock polymer 216 of triblock copolymer 818, rather than with the end blocks.
Chemists have proposed four general theories to explain the effects that plasticizers have on certain materials. These theories are known as the lubricity theory, the gel theory, the mechanistic theory and the free volume theory.
The lubricity theory, illustrated in FIGS. 9b through 9d, assumes that the rigidity of a material (i.e., its resistance to deformation) is caused by intermolecular friction. Under this theory, plasticizer 910 lubricates the large molecules, facilitating movement of those molecules over each other. See generally, Jacqueline I. Kroschwitz, ed., CONCISE ENCYCLOPEDIA OF POLYMER SCIENCE AND ENGINEERING 734-44, Plasticizers (1990), which is hereby incorporated by reference. In the case of triblock copolymers, lubrication of the endblocks should be avoided since the endblock domains are responsible for holding the triblock copolymers together and provide the material with strength (e.g., tensile strength during elongation). Thus, a plasticizer which associates with the midblocks is preferred. According to the lubricity theory, when manipulative force is exerted on the material, plasticizer 910 facilitates movement of midblocks 216 past each other. Id. at 734-35. The arrows in the Figures represent the motion of midblocks 216 with respect to each other. FIG. 9b represents adjacent midblocks being pulled away from each other. FIG. 9c represents two midblocks being forced side-to-side. FIG. 9d represents adjacent midblocks being pulled across one another.
FIGS. 9e and 9f depict a second plasticization theory, the gel theory, which reasons that the resistance of amorphous polymers to deformation results from an internal, three-dimensional honeycomb structure or gel. Loose attachments between adjacent polymer chains, which occur at intervals along the chains, called attachment points, form the gel. Closer attachment between adjacent chains creates a stiffer and more brittle material. Plasticizers 910 break, or solvate, the points of attachment 914 between polymer chains, loosening the structure of the material. Thus, plasticizers produce about the same effect on a material as if there were fewer attachment points between polymer chains, making the material softer or less brittle. See Id. at 735. Since one of the purposes of the present invention is to provide a material with improved tensile strength, which is provided by agglomeration of the endblocks, according to the gel theory plasticizer 910 should associate with midblocks 216 rather than with the endblocks. Further, a plasticizer which associates with the midblock polymers decreases the attachment of adjacent midblocks, which likely decreases the rigidity while increasing the pliability, elongation and elasticity or visco-elasticity of the material. Similar to the lubricity theory, under the gel theory, reduction of attachment points between adjacent midblocks facilitates movement of the midblocks past one another as force is applied to the material.
Referring now to FIG. 9g, the mechanistic theory of plasticization assumes that different types of plasticizers 910, 912, etc. are attracted to polymer chains by forces of different magnitudes. In addition, the mechanistic theory supposes that, rather than attach permanently, a plasticizer attaches to a given attachment point only to be later dislodged and replaced by another. This continuous exchange of plasticizers 910, 912, etc., demonstrated by the Figure as different stages connected by arrows which represent an equilibrium between each stage, is known as a dynamic equilibrium between solvation and desolvation of attachment points between adjacent polymer chains. The number or fraction of attachment points affected by a plasticizer depends upon various conditions, such as plasticizer concentration, temperature, and pressure. See Id. Accordingly, as applied to the material of this invention, a large amount of plasticizer would be necessary to affect the majority of midblock attachment points and thus provide the desired amounts of rigidity, softness, pliability, elongation and elasticity or visco-elasticity.
With reference to FIGS. 9h through 9j, the fourth plasticization theory, known as the free volume theory, assumes that there is nothing but free space between molecules. As molecular motion increases (e.g., due to heat), the free space between molecules increases. Thus, a disproportionate amount of that free volume is associated with the ends of the polymer chains. As FIGS. 9h through 9j demonstrate, free volume is increased by using polymers with shorter chain lengths. For example, the black rectangles of FIG. 9h represent a material made up of long midblock polymers 216. The white areas around each black rectangle represent a constant width of free space around the molecule. In FIG. 9i, a molecule 916, which is smaller than midblock 216, is added to the material, creating more free space. In FIG. 9j, an even smaller molecule 918 has been added to the material. The increase in free space within the material is evident from the increased area of white space. The crux of the free volume theory is that the increase in free space or volume allows the molecules to more easily move past one another. In other words, the use of a small (or low molecular weight) plasticizer increases the ability of the midblock polymer chains to move past each other. While the Figures provide a fair representation of the free volume theory, in reality, the increase in free space would be much greater since molecules are three-dimensional structures.
Similarly, the use of polymers with flexible side chains create additional free volume around the molecule, which produces a similar plasticization-like effect, called internal plasticization. Applicant believes that incorporation of monomers into the midblock, which create flexible side chains thereon, including but not limited to isoprene and ethylene/propylene monomers, creates internal plasticization. In comparision, the addition of an even smaller plasticizer molecule, described above, increases the free space at a given location; this is external plasticization. The size and shape of plasticizing molecule and the nature of its atoms and groups of atoms (i.e., nonpolar, polar, hydrogen bonding or not, and dense or light) determines its plasticizing ability on a specific polymer. See Id.
A visco-elastic material which deforms under pressure and has a high level of memory is needed. A material is also needed which has a slow rebound rate. In particular, a visco-elastic material is needed which has a tailorable rebound rate. A material with variable adhesive properties and stiffness is also needed.
With this general background in mind, Applicant will explain the formulation, chemical structure and performance of the invention.